Low density polyethylene (LDPE), i.e. a polyethylene with a density range of 910-940 kg/m3, is an important thermoplastic polymer and is of practical use in many industrial applications. Conventional low density polyethylene is produced by a high pressure process at a high temperature via free radical polymerization. Autoclave and tubular reactors are the two types of high pressure reactors which are predominantly used for producing low density polyethylene.
Further, in extrusion coating a thin film of polymer melt is extruded through a flat die and pressed onto a moving substrate. Extrusion coating is discussed, among others, in Vieweg, Schley and Schwarz: Kunststoff Handbuch, Band IV, Polyolefine, Carl Hanser Verlag (1969), pages 412 to 420. The substrate may be, among others, paper, paperboard, a plastic film or a metal film. The line speed in modern equipment can often be above 300 m/min or above 350 m/min.
Higher line speed sets heavy requirements for the material. Especially draw resonance is a problem often encountered with linear polymers, such as linear low density polyethylene (LLDPE), high density polyethylene (HDPE) or polypropylene (PP). At the onset of draw resonance large oscillations occur in the flow of the polymer melt through the die. Consequently, the coating becomes uneven. The draw resonance is due to the “tension thinning” behaviour of the linear polymers, where the elongational viscosity decreases with increasing tensile strain rate. On the other hand, highly branched polymers like low density polyethylene exhibit strain hardening, where the elongational viscosity increases with increasing tensile strain rate.
The two most important variables defining the processability of a polymer used for extrusion coating is its draw-down (DD) and neck-in (NI). The value of draw-down should be as high as possible in order to obtain as thin as possible coating layer and to allow a high production speed. At the same time it is desirable to have polymers with a low neck-in value. This first of all leads to a broader covering of the substrate, but also to less needs to trim away the outer part of the coated substrate. The latter is related to a phenomenon giving thicker edges of the melt film, “edge-bead”. With increasing neck-in this thickening will increase and a larger part of the polymer and substrate must be trimmed away. Further web stability at high line speeds is critical for obtaining extrusion coated surfaces with even coating weight.
Traditionally, the autoclave materials, here low density polyethylenes produced in a stirred autoclave reactor, have superior processability for extrusion coating together with satisfactory end product properties. The autoclave materials exhibit a pronounced high molecular weight tail and have a good neck-in draw down balance. Tubular materials, here low density polyethylenes produced in a tubular reactor, have hithereto, due to plug flow in the reactor, not shown such a pronounced high molecular weight tail, which is usually found in materials produced in autoclave reactors. Thus, the tubular materials have hithereto not shown a good neck-in draw down balance. Especially the neck-in will be high with a tubular material and web stability will also be lower. In order to have a advantageous neck-in draw down balance and a web stability at high line speeds, the tubular material must have a high storage modulus G′, measured at a loss modulus G″ of 5 kPa.
Further, since autoclave plants are getting older, and there are not many new autoclave reactors being built in the world there is a need for alternative technology giving same processability. However, as described above, traditionally produced tubular LDPE polymers have not fulfilled the requirements set for processability. Thus, there is a need for new polymer structures from tubular reactor with advantageous properties to meet the requirements of draw-down and neck-in and web stability.